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Lewis Structures' Tutorial

Part 3: Resonance


In the previous section we went through the rules for Lewis structures and multiple bonds. You may have noticed that the structures we worked through only had 2 atoms. Now, we'll work out how to do similar structures for molecules that have more than 2 atoms.

Pull up the Periodic Table if you need one.

Here's a current list of the rules.

How about the nitrate ion, NO31- ion? Work through the Lewis structure up to the point of making any multiple bonds.

NO31-

Your turn- enter your answer in the first box and hit "Verify" to see how you did.
1. Total number of valence electrons.
2. Draw the skeleton structure.
Nitrate ion skeleton
3. Connect the atoms by making bonds.
Nitrate ion bonds
4. Satisfy the octet rule for the surrounding atoms.
Nitrate ion's surrounding oxygens' octet

At this point you need to satisfy the octet rule for the central atoms. And, since there are no more electrons and you are short two, you need to share another pair of electrons from one of the oxygen atoms. Which one? Is there a correct oxygen to pick for sharing more electrons with nitrogen?

In other words, which of the following is the correct Lewis structure?

Nitrate ion resonance structure or Nitrate ion resonance structure or Nitrate ion resonance structure

Remember, as was discussed earlier, there can be different ways to represent the skeleton structure. That's not the question. Using the skeleton structure that I chose, which is the correct one?

Fortunately and unfortuntately, all three are correct. I say unfortunately because these multiple possibilities emphasize a limitation of Lewis structures. In the previous cases of single and multiple bonds, the bonding electrons were shared between the two atoms involved in the bond. Another way to describe it is that the bonding electrons were localized to two atoms. Now, the double bond between nitrogen and and oxygen atom is shared among 3 nitrogen-oxygen bonds. Or, the bonding electrons are delocalized among the 4 atoms involved in the bonding.

When you are asked to draw the Lewis structure for the nitrate ion, NO31-, any of the three will suffice. If you are asked to draw all of the resonance structures for NO31- then you need to draw all the possible ones in the following way with the arrows):

Nitrate ion resonance structure arrow Nitrate ion resonance structure arrow Nitrate ion resonance structure

You'll note that the skeleton structure does not change when you draw the possible resonance structures. Just the location of the multiple bonds.

Resonance also affects the bond order. Previously, the bond order for a double bond was assigned the value 2. What happens now since the double bond apparently is being shared three ways? Let's look back at the formal definition for bond order. It's the number of pairs of electrons shared among the two atoms in the bond. Since the bonding electrons are shared among multiple atoms (as in more than two), the way to calculate the bond order in resonance structures is to average it out among all the resonance-involved bonds.

So, for NO31- there is one bond that has a bond order of 2 (the double bond) and two bonds that have a bond order of 1 (the single bonds). This being a resonance structure and thus the double bond is shared among the nitrogen and the 3 oxygen atoms, the bond order is 4/3. The 4 because there are 4 pairs of electrons being shared and the 3 because there are 3 places for bonds (or 3 pairs of bonding atoms).

We need to modify our list of rules to allow for resonance structures. Remember that you only need to draw the multiple structures if you are asked in some way to draw all the possible resonance structures.

Next up is the ozone molecule, O3. Although you won't normally be told about multiple bonds and any needed resonance structures, this one does have resonance structures. Go ahead and work everything through including the resonance structures and bond order.

O3

Your turn- enter your answer in the first box and hit "Verify" to see how you did.
1. Determine the total number of valence electrons.
2. Draw the skeleton structure.
Ozone skeleton
3. Connect the atoms by making bonds.
Ozone bonds
4. Satisfy the octet rule for the surrounding atoms.
Ozone's surrounding oxygens' octet
5. Satisfy the octet rule for the central atom and all the possible resonance structures.
Ozone's resonance structures
6. Calculate the bond order.


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